Decanters, which are also called solid-bowl screw-type centrifuges, are employed to separate what are called suspensions, materials that comprise a mixture of solids and one or more liquids of different weights. Since the suspensions are separated by centrifugal force, high speeds can be necessary. The solids accumulate subject to gravity on the inner surface of the bowl, whence they are conveyed by a screw that rotates relatively slowly inside the rapidly rotating bowl, generally through a dry section that narrows conically up along the axis of rotation to what is called the solids-extraction point, whereas the liquid phase or the several liquid phases of different weights are extracted, generally at the end of the drum that is axially opposite the solids-extraction point, in batches with specific gravities that decrease successively toward the axis. Decanters of this type are known.
A decanter, especially a high-speed decanter, comprises in conjunction with its drive mechanism and sometimes with such accessories as a switchbox etc., all mounted on a joint machine bed, an instrumentation system with at least one critical natural frequency for the decanter's operating behavior that can be one of several natural frequencies in the system. The critical natural frequency, at which the moving parts oscillate with a high amplitude, is disruptive when it is near or below the decanter's operating frequency, which is dictated by the rotating parts: the drum, the screw, the motor, etc. The operating frequency is only theoretically constant and actually represents a particular range, affected even by the amount of solids taken in, by the controls, etc. Also present are vibration phenomena produced by the solids as a function of their connecting the drum to the screw to the extent that friction deriving from the screw that is advancing the solids leads to braking phenomena that express themselves as tension in the differential-speed transmission between the drum and the screw, so that the screw does not remain constant with respect to its differential speed in relation to the drum but is subjected to a particular rhythm in its variation of the speed.